1. Field of the Invention
The present invention relates to golf balls having a core, a cover and dimples.
2. Description of the Related Art
For golf balls, flight performance is important. Flight performance depends on aerodynamic characteristic of the golf ball. Aerodynamic characteristic heavily depends on specifications of dimples. The dimples disrupt the air flow around the golf ball during flight to cause turbulent flow separation. By causing the turbulent flow separation, separating points of the air from the golf ball shift backwards leading to the reduction of drag. The turbulent flow separation prolongs the gap between the separating point on the upper side and the separating point on the lower side of the golf ball, which results from the backspin, thereby the lift force that acts upon the golf ball is enhanced. Reduction in drag and elevation of lift force are referred to as “dimple effect”. Excellent dimples disturb the air flow more efficiently. Owing to the excellent dimples, great flight distance can be achieved.
It is known to persons skilled in the art that a great dimple effect is achieved according to golf balls having the dimples densely arranged. Some proposals have been made in connection with dimple pattern aiming at improvement of the dimple effect.
JP-A-50-8630 (U.S. Pat. Nos. 4,729,861, 4,936,587 and 5,080,367) discloses a golf ball provided with numerous dimples having a uniform size. In this golf ball, pitch is smaller than 0.065 inch for most of the dimple pairs. According to this golf ball, relationship between the pitch and dimple diameter is not considered. In comparison with general dimple diameter, the pitch of 0.065 inch is not small enough. According to the pattern of the dimples having a uniform size, the diameter can not be set to be great. The dimples in this golf ball are not arranged densely enough.
JP-A-62-192181 (U.S. Pat. No. 4,813,677) discloses a golf ball provided with large dimples and small dimples. In this golf ball, high dimple density is achieved by arranging small dimples in the region surrounded by multiple large dimples. However, the small dimples are not sufficiently responsible for the dimple effect.
JP-A-4-347177 (U.S. Pat. No. 5,292,132) discloses a golf ball having the dimples arranged so that any rectangle having a predetermined size can not be formed on the land. In this golf ball, small proportion of the land is achieved by arranging many small dimples. However, the small dimples are not sufficiently responsible for the dimple effect.
Flight performance also depends on deformation behavior of golf balls. Golf balls which are greatly deformed when being hit by a driver are excellent in resilience performance. Owing to the excellent resilience performance, great flight distance is obtained. When golf balls whose rigidity in shear direction is great are hit by a club, spin rate of the golf balls is low. Owing to the low spin rate, great flight distance is obtained. Especially when golf balls are hit by an iron club, spin rate heavily depends on rigidity in shear direction. In light of flight performance, proposals concerning improvement of the structure and materials of golf balls have been made.
The golf players place great importance also on spin performance of the golf balls. Great back spin rate results in small run. For golf players, golf balls which are liable to be spun backwards are apt to be rendered to stop at a targeted position. With high side spin rate, trajectory of the golf ball is easily curved. For golf players, golf balls which are liable to be spun sidewise are apt to allow their trajectory to curve intentionally. The golf balls that are excellent in spin performance are excellent in control performance. High class golf players particularly place great importance on control performance upon shots with a short iron. In light of flight performance and control performance, some proposals concerning improvement of structure and material of the golf ball have been made.
JP-A-9-239068 (U.S. Pat. No. 5,782,707) discloses a golf ball having a core, a mid layer and a cover whose hardness is less than that of the mid layer. In this golf ball, the cover deteriorates resilience performance. In this golf ball, spin rate is high when the golf ball is hit by an iron club. This golf ball is inferior in flight performance.
JP-A-2005-168701 (US Publication No. 2005/130768) discloses a golf ball having a cover which includes thermoplastic elastomers. This golf ball has large dimples. In this golf ball, the cover deteriorates resilience performance. In this golf ball, high spin rate is obtained when the golf ball is hit by an iron club. In this golf ball, there is room for improvement of the dimple pattern.
JP-A-2001-145709 (U.S. Pat. No. 6,319,154) discloses a golf ball having a core with predetermined hardness and a cover with predetermined hardness. In this golf ball, high spin rate is obtained when the golf ball is hit by a driver. This golf ball is inferior in flight performance.
Concern of golf players for golf balls is their flight distance and control performance. In light of flight performance, there is room for improvement of golf balls. An object of the present invention is to provide a golf ball that is excellent in flight performance. A further object of the present invention is to provide a golf ball that is excellent in control performance.